Normally Mystery snails will concentrate on algae and any other fish food in your aquarium - BUT - they WILL eat plants if they're hungry enough.
I've seem then devour all the plants in a friends planted aquarium in a matter of days!!
I used to keep and breed mystery snails and found the following to be great foods to feed:
- Grren Peas (Without the skins)
- Lettuce Leaves
- Poached Pumpkin
- Poached Carrot
- Zuccini
- Algae Wafers
- Fish food -either flakes or pellets dosen't really matter - as long as it sinks